Infrared active phonon modes and ionicity of single crystal MgAl2O4

Near-normal incident infrared reflectivity spectra of （100） MgAl2O4 spinel single crystal have been measured at different temperatures in the frequency region between 50 and 6000 cm^-1. Eight infrared-active phonon modes are identified, which are fitted with the factorized form of the dielectric function. The dielectric property and optical conductivity of the MgAl2O4 crystal are analysed. From TO/LO splitting, the effective Szigeti charges and Born effective charges at different temperatures are calculated for studying the ionicity and the effect of polarization. Based on the relationship between the （LO-TO）1 splitting, which represents the transverse and longitudinal frequencies splitting of the highest energy phonon band in the reflectivity spectrum, and the ionic-covalent parameter, the four main phonon modes are assigned. MgA1204 can be considered as a pure ionic crystal and its optical characters do not change with decreasing temperature, so it may be used as a suitable substrate for high-Tc superconducting thin films.     

First-principles study of the dielectric properties and infrared reflectance spectrum of Y2O3

Yttrium oxide is an important laser and infrared optical material. The structural, vibrational and dielectric properties of Y ₂O₃ are calculated from first principles using the plane-wave pseudopotential method. The dielectric permittivity tensors, infrared-active phonon frequencies at the Brillouin zone center and the LO/TO splitting are reported within the framework of density functional perturbation theory. Contributions to the static dielectric constant from each infrared-active mode are presented. It is shown that Y ₂O₃ has an electronic dielectric constant larger than that of the lattice contributions. Dielectric, refractive index, extinction coefficient and infrared reflectance spectra of Y ₂O₃ are given, and the figures suggest that Y ₂O₃ presents good transmission properties in the spectrum range above 800 cm^?1 or below 400 cm^?1.     

Opto-electronic response of spinels MgAl2O4 and MgGa2O4 through modified Becke-Johnson exchange potential

A first-principles technique capable of describing the state accurately near to excited states of semiconductors and insulators, namely the modified Becke-Johnson (mBJ) exchange potential approximation is used to investigate the opto-electronic response of magnesium spinel oxides: MgAl₂O₄ and MgGa₂O₄. The predicted bandgaps using the mBJ exchange approximation show a significant improvement over previous theoretical work using the common LDA and GGA, and are very closer to the experimental results. Band gap dependent optical parameters, like dielectric constant, index of refraction, reflectivity and optical conductivity are calculated and analyzed.     

Vibrational and dielectric properties of La2Hf2O7 : Experiment and theory

The physical origin of the static dielectric constant and its relationship with lattice dynamics of La₂Hf₂O₇are studied by combining infrared spectroscopy and density functional perturbation theory (DFPT). Both La and Hf show obvious effective charge anomaly which is attributed to the hybridization between 2p states of the oxygen and 5d states of the cations, indicating a mixed covalent-ionic bonding between the cations and the oxygen. The dielectric response is determined by seven infrared phonon modes and the static dielectric constant extracted from infrared reflection spectrum is in close agreement with DFPT calculation. Both experiment and theory reveal that most of the contributions to the static dielectric constant are dominated by three infrared phonon modes at 137, 172 and 297 cm⁻¹. Two of them (172 and 297 cm⁻¹) are from the displacements of oxygen atoms inside HfO₆ octahedra and the other one (137 cm⁻¹) is from the bending of La₂O′ chain. This result indicates that the origin of the static dielectric constant of La₂Hf₂O₇ is directly connected with the two interpenetrating sub-networks of pyrochlore structure (HfO₆ octahedra and La₂O′ chain).     

(1-x)MgAl2O4-xTiO2 dielectrics for microwave and millimeter wave applications

The MgAl₂O₄ ceramics were prepared by the conventional solid-state ceramic route and the dielectric properties studied in the microwave frequency region (3–13 GHz). The phase purity and crystal structure were identified using the X-ray diffraction technique. The MgAl₂O₄ spinel ceramics show interesting microwave dielectric properties (ε_r=8.75, Q_uxf=68900 GHz (loss tangent = 0.00017 at 12.3 GHz), τ_f=-75 ppm/°C). The MgAl₂O₄ has high negative τ_f, which precludes its immediate use in practical applications. Hence the microwave dielectric properties of MgAl₂O₄ spinels were tailored by adding different mole fractions of TiO₂. The ε_r and Q factor of the mixed phases were increased with the molar addition of TiO₂ into the spinel to form mixtures based on (1-x)MgAl₂O₄-xTiO₂ (x=0.0-1.0). For x=0.25 in (1-x)MgAl₂O₄-xTiO₂, the microwave quality factor reaches a maximum value of Q_uxf=105400 GHz (loss tangent = 0.00007 at 7.5 GHz) where ε_r and τ_f are 11.035 and -12 ppm/°C, respectively. The microwave dielectric properties of the newly developed 0.75MgAl₂O₄-0.25TiO₂ dielectric is superior to several commercially available low loss dielectric substrates. PACS 77.22.-d; 84.40.-x; 81.05.Je     

Optical spectrum and local lattice structure for ruby

We perform ab initio calculations using a pseudo-potential plane-wave method based on density functional theory, within the local density approximation and generalized gradient approximation, in order to determine and predict the pressure dependence of structural and elastic properties of spinel compounds: MgAl₂O₄, MgGa₂O₄ and MgIn₂O₄. The results are in agreement with the available experimental data and other theoretical calculations.     

First-principles calculations of the dielectric and vibrational properties of ferroelectric and paraelectric BaAl2O4

First-principles calculations have been conducted to study the structural, dielectric, and vibrational properties of ferroelectric and paraelectric BaAl₂O₄. High-frequency and static dielectric constants, and phonon frequencies at the Brillouin zone center are reported. Both BaAl₂O₄ polymorphs are promising infrared-transparent materials due to their low electronic dielectric constants. The ferroelectric and paraelectric BaAl₂O₄ have much smaller permittivity compared to the classical ferroelectric materials. From an atomic nanostructure standpoint, the abnormally low permittivity of BaAl₂O₄ polymorphs is mainly related to low coordination numbers of Ba (9) and Al (4).     

Studies of the electronic and optical properties of BaMxO1−x (M = S, Se, Te) using first-principle calculations

The electronic and optical properties of BaM_xO_1−x (M = S, Se, Te) have been studied by using the density functional theory implemented with the GGA-PW91 method. The calculated results show that the lattice constant of BaM_xO_1−x obeys the Vegard model while the energy gap of BaM_xO_1−x shows a different trend with the changing molar fraction x. The optical properties of the BaM_xO_1−x compounds with the physical quantities such as the complex dielectric function, the energy loss function and the static dielectric constant, respectively, are given to support the potential applications of the compounds in the future.     

Hydrogen storage: Lattice dynamics of orthorhombic NaMgH3

In this work, we investigate the structural, dynamic and thermodynamic properties of NaMgH₃, devoted for hydrogen storage. Density functional theory using pseudopotential methods and generalized gradient approximation has been used. A good agreement between the calculated structural parameters and the experimental data was found. A linear-response approach for the density functional theory is used in order to derive the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the center of the Brillouin zone, the phonon-dispersion curves and the corresponding density of states for NaMgH₃ material. The obtained phonon frequencies at the zone center (Γ point) for the Raman-active and infrared-active modes are analyzed. Thermodynamic functions using the phonon density of states are also calculated.     

Estimation of the influence of ionic dielectricity on the dynamic superconducting order parameter

We consider the influence of an ω-dependent ionic dielectric constant ?(ω) on the properties of a superconductor. Assuming that the pairing interaction is proportional to ?² we have solved the Eliashberg equations for this case, both for imaginary and real frequencies. The interaction potential depends on a coupling constant λ and on a longitudinal phonon frequency Ω. The dielectric constant is assumed to be independent of wavevector q, and to depend on frequency through the expression: ?(ω) = (ω² - ω²_long)/(ω² - ω²_trans), where ω_long, ω_trans are the frequencies of optical phonons of the dielectric. We find that along the imaginary frequency axis (but not for real frequencies) the weighted phonon propagator can be modeled by an appropriate choice of a cutoff frequency and an effective coupling constant. The influence of ?(ω) on T_c, the gap δ(ω), and the renormalization function Z(ω) are studied and it is found that these quantities increase significantly with the dielectric constant.     

Ab initio vibrational and dielectric properties of chalcopyrite CuInS <Emphasis Type="Bold">2</Emphasis>

We have performed a first-principles study of structural, dynamical, and dielectric properties of the chalcopyrite semiconductor CuInS₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. Born effective charge tensors, dielectric permitivity tensors, the phonon frequencies at the Brillouin zone center and mode oscillator strengths are calculated using density functional perturbation theory. The calculated properties agree with infrared and Raman measurements. Received 12 December 2002 / Received in final form 17 March 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: resul@ibu.edu.tr     

Volume dependence of the A1g phonons in YBa2Cu3O7

By employing first-principles frozen-phonon calculations we have determined the force-free geometry and the dynamical matrix for the five Raman-active A1g modes in a volume-reduced YBa₂Cu₃O₇ crystal. By this mean we investigate the relation between the effect of overbinding caused by the local density approximation and the vibronic properties of this material. This procedure reveals noticeably improved phonon frequencies in comparison to the theoretical results obtained for the measured lattice constants.     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

Vibrational, dielectric and scintillation properties of YAlO3

Vibrational and dielectric properties of YAlO₃ are investigated within the framework of density functional perturbation theory. The calculated zone center phonon frequencies and dielectric constants are in good agreement with available experimental data. Based on the theoretical values of the dielectric constants and the highest longitudinal IR phonon energy and using the phenomenological model of Lempicki and Wojtowicz, we investigate the scintillation properties of the YAlO₃.     

High T c superconductors as ionic metals and the role of phonons in high T c superconductivity

We have investigated a number of zone center and zone boundary (X-point) modes in La₂CuO₄ using the full potential Linearized Augmented Plane Wave (LAPW) method and find excellent agreement with experiment for phonon frequencies, showing that the local density approximation (LDA) is an excellent approximation for the total energies, charge density, and static density response in high T _c superconductors. Results are compared with those of a non-empirical ionic model for La₂CuO₄ and YBa₂Cu₃O₇. Self-consistent calculations show that the ionic nature of these materials leads to large changes in the potential with atomic displacements. The ionic contributions to the electron-phonon matrix elements, which were neglected in previous studies, are large and possibly sufficient to explain high T _c .     

(100) MgAl2O4 as a lattice-matched substrate for the epitaxial thin film deposition of the relaxor ferroelectric PMN-PT

The (100) surface of MgAl₂O₄ is evaluated as a substrate for the thin film deposition of the relaxor ferroelectric PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%). With a lattice mismatch of less than 0.5%, this film-substrate combination presents a geometrical template for growth that is far superior to that formed with other commercially available oxide substrates. Films were deposited using the pulsed laser deposition technique and were characterized in terms of their crystallographic, microstructural, and dielectric properties. From a crystallographic perspective the films show excellent cube-on-cube epitaxy, are highly oriented, and show no evidence of the frequently observed parasitic pyrochlore phase. With the exception of a few faceted surface structures, the film’s microstructure is single-crystal-like, exhibiting a sharp film-substrate interface, a smooth top surface, and no discernable granularity. The dielectric response shows the frequency-dependent diffuse phase transition characteristic of a relaxor material, but with less frequency dispersion and a smaller maximum in the dielectric constant. Taken together, the results suggest that the (100) MgAl₂O₄ substrate could prove to be an effective substrate material, not only for the PbMg_1/3Nb_2/3O₃(65%)–PbTiO₃(35%) system, but also for a number of other important lattice-matched ferroelectric, relaxor, and ferroelectric superlattice systems.     

Ab initio calculation of the lattice dynamics of the Boron group-V compounds under high pressure

High-pressure effects on the lattice dynamics and dielectric properties of the BN, BP, BAs, BSb and BBi alloys have been carried out using the density-functional perturbation theory within the local density approximation. We study the variation of the optical phonon frequencies (ω_TO and ω_LO), the high-frequency dielectric coefficient (ε_∞) and the dynamic effective charge (Z*) with pressure. The ω_TO and ω_LO have a quadratic form with pressure for all boron compounds. The obtained ε_∞ and Z* for BN, BP remain constant with pressure. However, for BAs, BSb and BBi, ε_∞ and Z* have a quadratic form with pressure. Our results are in good agreement with the available experimental data for BN and BP and they allow prediction for BAs, BSb and BBi.     

First principles study of electronic structure and optical properties of double perovskite Ba2(InM)O6 [M=Nb,Ta]

The structural, electronic and optical properties of double perovskite oxides Ba₂(InNb)O₆ and Ba₂(InTa)O₆ are investigated by full-potential linearized augmented plane wave method based on density functional theory under generalized gradient approximation. The real and imaginary parts of the dielectric function and the optical constant (refractive index) are calculated. The calculated spectra are compared with the experimental results and are found to be in reasonable agreement. The interband contributions to the optical properties of these perovskite oxides have been analyzed.     

A1 g phonons in YBa2Cu3O7 by first-principles atomic-force calculations

We report on ab-initio frozen-phonon calculations for the five A_1g Raman-active-modes of YBa₂Cu₃O₇ atq=0. The determination of the equilibrium positions of the atoms Ba, Cu(2), O(2), O(3), O(4) as well as of phonon frequencies and respective eigenvectors is based on the calculation of atomic forces within the LAPW-method. The general agreement between our results and experimental structural data and phonon frequencies is good. Deviations are discussed in connection with the local-density approximation.     

Dielectric properties and magnetoelectric effect of (x)NiFe2O4+(1−x)Ba0.8Sr0.2TiO3 composites

Magnetoelectric composites of NiFe₂O₄ and Ba_0.8Sr_0.2TiO₃ were prepared using conventional double-sintering ceramic method. The phase formation of magnetoelectric composites was confirmed by XRD technique. Variation of dielectric constant and loss tangent at room temperature with frequency in the range 100 Hz-1 MHz has been studied. Also the variation of dielectric constant and loss tangent with temperature and composition at fixed frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz is reported. The static value of the magnetoelectric conversion factor was measured as a function of intensity of the magnetic field. The ME voltage coefficient of about 430 μV/cm Oe was observed for 15% NiFe₂O₄+85% Ba_0.8Sr_0.2TiO₃ composite. All the samples show linear variation of magnetoelectric conversion in the presence of static magnetic field.